In the prior art, CMOS (Complementary Metal Oxide Semiconductor) transistors which are elementary electronic elements are commonly used in a variety of electronic products. A CMOS transistor is composed of two complementary MOS transistors including an NMOS transistor and a PMOS transistor. In order to manufacture the PMOS transistor, firstly N-type ions are implanted on a silicon substrate to form an N-well area, and then pattern layers of gate, source and drain are manufactured in the N-well area.
The resistance of the N-well is an important electrical parameter of the CMOS transistor, and the stability in a value of the resistance of the N-well is influenced directly by the concentration of the ions in the N-well area.
The two MOS transistors of the CMOS transistor are isolated from each other by a field oxide layer which is manufactured in a procedure divided into two steps of well driving-in and oxidation. In the prior art, in the procedure for manufacturing the field oxide layer, a thin oxide layer is formed through dry oxidation before the step of well driving-in.
However, the inventors have identified, during making of a technical solution in embodiments of the invention, at least the following technical problems in the prior art.
In the prior art, in the procedure for manufacturing the field oxide layer, the thin oxide layer is formed through dry oxidation. The thickness of the thin oxide layer is not sufficient to prevent effectively the N-well from being contaminated due to ions introduced thereto in the manufacture procedure.
In the prior art, poor stability or another abnormality of a device or a process in the manufacture procedure may cause ion contamination, where the ions remaining on the surface of the field oxide layer may penetrate the field oxide layer to be diffused into the well area below the field oxide layer, thus having the concentration of ions in the area changed and consequently the resistance of the N-well of the CMOS transistor changed and even exceeding a prescribed parameter, which may be greatly adverse to the reliability and the electrical stability of the product.